Pressure-casting machine

ABSTRACT

A pressure casting machine includes a chill mold, mold-closing means, a filling bush opening into the mold and a pressing ram displaceable in the filling bush. A hydraulic pressing unit is associated with the ram and includes a shot cylinder, a hydraulically operated shot piston, and a multiplier cylinder connected with the shot cylinder and having a multiplier piston which is hydraulically loadable on both sides and hydraulically connectable to the shot piston. An accumulator is chargeable by a pressure medium source, and hydraulic components interconnect the pressure medium source, the accumulator, the shot cylinder and the multiplier cylinder. A hydraulic line connects the multiplier cylinder, on the side of the multiplier piston facing the pressing ram, to the pressure medium source. An overflow valve is interposed in this line and is adjustable as to operating pressure. A controllable shutoff valve is connected to the overflow valve and acts as a precontrol for the overflow valve. The overflow valve is in the range of influence of the hydraulic pressure effective on that side of the shot piston facing away from the pressing ram. A switching device, operable in dependence on the pressure, is in controlling connection with the shutoff valve and operative to effect hydraulic connection of the multiplier piston and the shot piston in accordance with the operating pressure of the overflow valve and within the pressure range limited by the accumulator. The pressure-responsive control means may be purely hydraulic or may be a combination of electrically and hydraulically operated components.

United States Patent {72] Inventors Alfred Net; Primary Examiner-J. Spencer Overholser Siegfried Erhard, both of Uzwil, Assistant ExaminerJohn E. Roethel Switzerland AHOrneyMcGlew and Toren [21} Appl. No. 860,557 {22] Filed Sept. 24, 1969 45] patented Aug. 24, 1971 ABSTRACT: A pressure casting machine includes a chill '23 Assignce Gebmeder Buehkr c, mold, mold-closing means, a filling bush opening into the u fl s iu l d mold and a pressing ram displaceable in the filling bush. A I 32} priority Sept 30 9 hydraulic pressing unit is associated with the ram and includes {331 Switzerland a shot cylinder, a hydraulically operated shot piston, and a 9 53 multiplier cylinder connected with the shot cylinder and having a multiplier piston which is hydraulically loadable on both sides and hydraulically connectable to the shot piston. An accumulator is chargeable by a pressure medium source, and hydraulic components interconnect the pressure medium source, the accumulator, the shot cylinder and the multiplier cylinder. A hydraulic line connects the multiplier cylinder, on the side of the multiplier piston facing the pressing ram, to the 5 PRESSURECASTING MACHINE pressure medium source. An overflow valve is interposed in 11 Claims, 4 Drawing Figs. this line and is adjustable as to operating pressure. A controllable shutoff valve is connected to the overflow valve and acts [52] US. Cl... 164/315 as a precontrol for the overflow valve. The overflow valve is in [51] Int. Cl; 822d 17/10 the range of influence of the hydraulic pressure effective on [50] Field of Search 164/315 that side of the shot piston facing away from the pressing ram. A switching device, operable in dependence on the pressure, (56] References Cited is in controlling connection with the shutoff valve and opera- UNITED STATES PATENTS tive to effect hydraulic connection of the multiplier piston and the shot piston in accordance with the operating pressure of 2,585,297 2/1952 Beuscher 164/315 the overflow valve and within the pressure range limited by 2.620.528 12/1952 DeSternberg 164/315 the accumulator, The pressure-responsive control means may 2.634.468 4/1953 Holder 164/315 be purely hydraulic or may be a combination of electrically 3,319,701 5/1967 Eggenberger 164/315 and hydraulically operated components.

////////////// Ill/1 llllllllLL/Ll l eg PRESSURE-CASTING MACHINE BACKGROUND OF THE INVENTION I In the operation of metal pressure casting machines, it has long been known how to exert, following filling of the mold and during the casting of the metal in chill molds, a very great afterpressure force on the injected melt by the shot ram, for the purpose of avoiding cracking and piping. This application of after-pressure, effected in the last stage, is often applied by the use of a multiplier operatively associated with the shot unit, this multiplier comprising a multiplier cylinder and a multiplier piston, the latter being hydraulically connectable to the shot piston and displaceable in the multiplier cylinder.

For connection of the multiplier to the shot unit, as well as for control of the multiplication, that is, of the pressure effectively acting on the shot piston and which is greater than the pressure supplied by the pump or, respectively, by the accumulator, various expedients have hitherto been proposed. Since it is most important that care must be taken that the multiplier piston does not, during advance of the shot piston in is adjustable as to operating pressure and is precontrollable by 1 a controllable shutoff .valve connected thereto. The overflow valve is in the range of influence of the hydraulic pressure filling the mold, execute any movement, in order to'have the" s'ideratum is that the magnitudeof the multiplication, that is, the effective-after-pressure, should be adjustable within the widest possible limits.

. If, as has been common practice, there is provided, between the rams side of the multiplier cylinder and the source of the pressure medium, only an overflow valve,such an arrangement has the disadvantages ofja very often greatly reduced multiplication, on the one hand, and of a dragging rise of the multiplier pressure on theother hand. The reason for the greatly reduced multiplication is that, to prevent a movement of the multiplier piston, this overflow valve must already be adjusted to a very high response pressure for effecting overflow, so that, in the end, only a very small multiplication is possible.

If oneuses an accumulator instead of a tank, as a variant of this knowndesign, then, above all, the multiplication is not more advantageous. For both known designs, moreover, there is the disadvantagethat the connection of the multiplication cannot be influenced in time.

If a controllable shutoff valve is used between the multiplier and the pressure medium tank, instead of an overflow valve, then the multiplier can be connected to the shot unit very quickly, but the work is always possible only with maximum multiplication.

SUMMARY OF THE INVENTION This invention relates to the pressure casting of metal melts and,'more particularly, to a novel and improved pressure casting machine with an improved control of the multiplied after-' pressureforce on the injected melt.

The objective of the invention is to meet the conditions mentioned above and, accordingly, to eliminate the disadvantages inherent in known machines. For this purpose, the

invention is directed to a pressure casting machine, for the casting of metal melts in'chill molds, comprising a mold-closing press, a filling bush opening into the chill mold, and a pressing ram displaceable in the filling bush. The pressing ram has operatively associated therewith a hydraulic pressing unit comprising a shot cylinder, a hydraulically loadable ,shot

- piston displaceable in the shot cylinder, a multiplier cylinder correlated with the shot cylinder and having a multiplier piston displaceable therein, the multiplier piston being hydraulically loadable on both sides and hydraulically conprevailing on that side of the shot piston facing away from the pressing ram. A pressure-dependently operable switching device is in controlling connection with the shutoff valve, and effective to bring about, at the same time, hydraulic connection of the multiplier piston to the shot piston in accordance with the pressure as set by the overflow valve within the pressure range limited by the accumulator. r

The advantage of this arrangement is that, regardless of the pressures prevailing during the preceding advance and moldfilling phases, the multiplieris hydraulically connectable to the shot piston through the entire pressure range of the accumulator. v

In accordance with another feature of'the invention, a throttle point, for the time-delayed reversal of the shutoff valve, and known per se, is provided in a connecting line extending between the pressure medium inlet for the shot piston and the shutoff valve. Further, there is connected, between this connecting line and the accumulator, a pressure vessel provided with mechanical separating means.

'As a further feature of the invention, and as a modification thereof, an electrically controllable shutoff valve, together with a hydroelectric pressure switch, may be provided for the precontrolledor anticipatory function of the overflow valve. This arrangement, in turn, has the advantage that, by reversal of the excitation of the electrically controllable shutoff valve as a function of the response of the hydroelectric pressure switch, and by a simple potentiometer adjustment, the blocked position of the multiplier piston, as well as the adjustment of the desired multiplication, are rendered, possible through corresponding actuation of the overflow valve by the electrohydraulic shutoff valve.

An object of the invention is to provide an improved pressure casting machine for casting metal melts in chill molds with a multiplier arrangement providing a very large afterpressure force on the injected melt.

Another object'of the invention is to provide such a pressure casting machine in which, regardless of the pressures prevailing during the preceding advance and mold-filling phases, the multiplier is hydraulically connectable to the-shot piston over the entire pressure range of a hydraulic accumulator.

A further object of the invention is to provide such a pressure casting machine in which a blocked position of a multiplier piston as well as adjustment of the desired multiplication can be effected in a simple manner.

An understanding of the principles of the invention, reference is made to the following description of typical embodiments thereof as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the Drawings: FIG. I is largely schematic front elevation view of a pressure casting machine, with a correlated pressing unit and as- FIG. 4 is a view, similar to FIGS. 2 and 3, illustrating the es sential control elements of a third embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the pressure casting machine therein comprises, as the most essential elements, a machine bed 1, a fixed mold and nozzle support 2, a movable mold support 3, a toggle support bracket 4, a mold closing cylinder S, a toggle joint 6, and guide rods 7 and 7'. Bracket or support 2 supports a fixed mold half 8, andbracket or support 3supports a movable mold half 9. A filling bush 10 is supported in bracket 2 and has displaceable therein a pressing ram 1 1 connected to a shot piston 'in a shot cylinder 12 aligned with a multiplier cylinder 13. A compressed gas tank 14 communicates, through .a line l5,'with a pressure medium accumulator 16. The control parts necessary for an understanding of the invention comprise a control cabinet 20, which contains principally electric control elements but which may be provided also with hydraulic control components. In addition, there is a tank or reservoir 30 for the pressure medium, a pump 31 connected to reservoir 30, and a reversing slide valve 32 connected to tank 30 and to pump 31. A connecting line 43 extends from multiplier cylinder 13 to a multiplier control 33, and an accumulator-connecting control line 45 extends from control cabinet 20 to a connecting block 70 associated with accumulator 16. Between connecting lines 42 and 43, connected to shot cylinder 12 and multiplier 13, respectively, there is a check valve 48 which is inserted in a connecting line 46.

FIG. 2 illustrates the hydraulic pressing unit in greater I detail, along with respective essential hydraulic connecting elements, control elements, or both. A displaceable piston 51 is arranged in accumulator l6, dividing the accumulator into a compressed gas chamber, to the left of piston 51, and a hydraulic pressure chamber, to the right of piston 51. A shot piston 62, which may have hydraulic pressure applied to both of its opposite side's, is displaceably arranged in shot cylinder 12- and includes a piston rod 63 connectedwith pressing ram 11. Multiplier cylinder 13 is operatively associated with shot cylinder 12, as by being axiallyaligned therewith, and has displaceable therein a multiplier piston 65 which also is arranged for application of hydraulic pressure to opposite sides thereof.

In addition, multiplier piston 65 is hydraulically connectable with shot piston 62. A check valve 67 is arranged in an axial bore 66 of multiplier piston65 and, in order to open check a valve 67 in the rest position of piston 65, a fixedmechanical which is secured to both accumulator 16 and multiplier cylinder 13, block 70 having an accumulator connecting line '71 formed therein. In block 70, and operatively associated with bore or line 71, there is a controllable check valve 75 which shuts off toward accumulator 16. The control line opens into control cylinder bore 76 beneath check valve 75 or on the side opposite check valve 75 away from the valve seat. Block 70 also has communicating therewith accumulator loading line 34 and shot line 36.

. in the connecting line 43 extending from multiplier cylinder 13, at that end thereof nearer to pressing ram 11, to tank 30,

In a branch line 93 extending. between connecting line 41, downstream of throttle member 90, and accumulator loading line 34, there is provided a pressure vessel 95 with a mechanical separating member 96 movable therein and which is subjected to hydraulic pressure on both of its opposite sides. Ad-

ditionally, the pressure vessel 95 is provided with a mechani-.

cal, displaceable path limitation 98 for separating member 96. The last-named connecting and control elements, especially the elements 90 and 95, form a switching device.

The operation'of the pressure casting machine, with the illustrated pressing unit and the respective control in accordance with the first embodiment of the invention, as shown in FIGS. 1 and 2, will now be described. Reversing valve 32 is initially in a position connecting line 42 to pump 31 and con- 1 necting line 36 to reservoir 30. By reversing valve 32 from this initial position into the opposite position, line 42 is connected to reservoir 30 and shot line 36 is connected with pump 31, thereby initiating the mold-filling process. This mold-filling process takes place after the two mold halves 8 and 9 have been closed by the mold-closing press formed by supports 2 and 3, toggle support bracket 4, guiderod 7, 7' and closing cylinder 5, with the filling of filling bush 10 with a metal melt to be cast. Without going into detail with the familiar further control elements for the control of the very slow advance'of pressing ram 11, due to the very low hydraulic admission of shot piston 62 in shot cylinder 12, or the triggering of the very rapid chill mold-filling advance, these are the first two operational phases.

Shot piston 62 moves from the illustrated position, at the extreme right, to the left. However, care must be taken that multiplier piston 65 does not immediately start a forward movement, that is a movement to the left. The avoidance of this is the function of overflow valve 80 together with shutoff valve 85, this avoidance occurring as a function of the pressures acting on shot piston 62 and prevailing in accumulator 16. During the prefilling phase, as well as during the rapid shot for mold filling, the pressure required therefor and acting on shot piston 62 remains substantially lower than the pressure prevailing in accumulator 16. Thus, shot off valve 85 remains in a position blocking flow from precontrol line 82 into line 42 extending to reservoir 30. Thereby, overflow valve 80 can be opened only with a maximum pressure, to establish a connection between multiplier cylinder 13, on the end thereof nearer pressing ram 11, and connecting element 42. Even if very great pressure were to act on the hydraulic medium enclosed in that volume of multiplier cylinder 13 facing the pressing in ram, a movement of multiplier piston 65 to the left would be prevented. The shot forfilling the mold, occurring at a very high velocity, can take place with the controllable check valve 76 open and thus utilizing the highly compressed pressure medium in accumulator 16 to the right of piston 51.

At the instant of final mold filling with the metal melt to be cast, the pressure acting on shot piston 62 increases extremely rapidly. This pressure now becomes effective through conthere is interposed an overflow valve which has a pressure adjustment .means 81 and which has connected thereto a precontrol line 82 and its necessary own control pressure line 83. Between precontrol line 82.and the line 42 extending to tank 30 or pump 31 there are connected a shutoff valve 85 and a check valve 88. For the control of shutoff valve 85 there serves, on the one hand, a hydraulic line 41 connected to valve 85 and also to shot line 36 and having a throttle member arranged therein and, on the other hand, a line 91 connected between valve 85 and accumulator line 34.

necting line 41 and throttle member 90, interposed in con necting line 41, into branch line 93. However, because of pressure vessel 95 and branch line 93, this pressure, at first, is substantially reduced during the then following displacement of piston 96 against stop 98, so that there still exists, upon shutoff valve 85, a slight pressure gradient which is insufficient to open shutoff valve 85.

Piston 96 in pressure vessel 95 is a differential piston, so that already, by a lesser pressure in the zone of shot piston 62, piston 96 is displaced against the pressure of accumulator 16. As soon as piston 96 strikes against mechanical stop 98, the pressure at shutoff valve 85 immediately increases to the pressure prevailing on shot piston 62, and this serves to reverse shutoff valve 85 into a position establishing a connection from overflow valve 80 through precontrol line 82 into line 42 leading to tank 30. As there now prevails, in precontrol line 82, the pressure prevailing in reservoir 30, and which is generally atmospheric pressure, overflow valve 80 also opens as a result of the very great pressure prevailing in multiplier cylinder 13,

and which can be as high as the pressure adjustment permits. This set pressure acts as a counterpressure on multiplier piston 65, and the remaining difference pressure is available for the multiplication. In this instant, by the hydraulic connection between multiplier piston 65 and shot piston 62, the latter has applied thereto the new pressure, that is, precisely the multiplier pressure. This effects an after-pressing of the metal melt to be cast and during the solidification period in the child mold 8, 9.

After completion of the standing time, required for cooling of the pressure cast part, reversing slide valve 32 is moved back into its illustrated position. Thereby pressure medium is supplied into both of the volumes of shot cylinder 12, that is, the volume toward the pressing ram and the volume toward multiplier cylinder 13. Multiplier piston 65 is moved back into the illustrated rest position, check valve 67 is opened and shot piston 65 moves toward multiplier piston 65, displacing pressure medium through the open check valve 67 and through shot line 36 now communicating with reservoir 30 whereas line 42 is communicating with pump 31. At the same time, check valve 75 in connecting block 70 is closed again and accumulator 16 simultaneously is hydraulically charged. Over flow valve 80 and shutoff valve 85 are either out of operation or in the blocking position.

In the embodiment of the invention shown in FIG. 2, throttle member 90, as well as pressure vessel 95, have been shown as including adjusting elements. Generally, it may suffice to make one of these elements adjustable since, in accordance with electrical engineering principles, for this combination,

which may be regarded as an R-C member, either the adjustability of the R member, for example the throttle 90, or the adjustability of the C member, for example the pressure vessel 95, is sufficient. In fact, when working with a permanently set throttle member 90, it is sufiicient to adjust the mechanical setting member 98 in pressure vessel 95 to vary the charge amount of hydraulic medium to release or trigger the pressure on shutoff valve 85. With this arrangement, there is obtained, in a very simple manner, a connectability of the multiplication pressure to shot piston 62 and which is variable in time, after the mold has been filled. The combination of shutoff valve 85 with overflow valve 80 results, in an exceedingly simple manner, in an absolute blocking of multiplier piston 65 during the casting phase until actual mold filling, on the one hand, and to any desired multiplication within the pressure limit range set by accumulator 16. Owing to this arrangement, it is possible to provide and adjust overflow valve 80, regardless of the prevailing pressures which differ very greatly in dependence on the chill mold and the casting parts, to the actual lower, by a constant amount, than the pressure prevailing in accumulator 16. The mode of operation of the embodiment of the invention shown in FIGS corresponds to that of the embodiment of the invention shown in FIGS. 1 and 2. The em-' bodiment ofthe invention shown in FIG. 3 has the advantage that the instrumentation includes, in part, simpler elements.

The third embodiment of the invention, shown in FIG. 4, has basically a hydraulic electric circuit arrangement. In contrast to the two embodiments already described, there is pro vided, instead of the purely hydraulic shutoff valve 85, a solenoid or electrically operated shutoff valve 115. For the pressure dependent control of shutoff valve 115, a pressure dependent reversing switch, 125 is used. Switch 125 includes a cylinder 126 in which there is slidable a piston 128 biased by a spring 127, to the left, and loadable, counter to the spring bias, by the hydraulic pressure acting on shot piston 62. The piston rod 129 of piston 128 extends through an axially adjustable bearing 130 and carries a contact closer 140. At least one rest contact pair 141 and at least one working contact pair 142, of an electric circuit, are operatively associated with circuit closer 140. I

The electric circuit includes a potential source 145, a time switch 146 and a variable resistance or potentiometer 147 having an adjustable tap 148. Tap 148 is connected with the operating winding 150 of the electrohydraulic shutoff valve 115, time switch 146 is connected with the working contacts 142 of switch 125, and rest contacts 141 of switch 125 are connected, in parallel with adjustable tap 148, to the operating winding 150 of shutoff valve 115. In contrast with the previously described embodiments, shot line 36' communicates directly with multiplier cylinder 13.

For the control shown in FIG. 4, it is advantageous to use an electrohydraulic valve 115 such as shown in French Pat. No. 1,472,397, as such an electrohydraulic valve is able to block off increasingly greater hydraulic pressures by the use of increasingly greater exciter current. As the function of shutoff valve 115 is-to block precontrol line 82, during the actual mold-filling phases and against the highest hydraulic pressures which become active from overflow valve 80, maximum excitation is also necessary for valve 115. Conversely, overflow valve 80 must be adjusted to a certain pressure during the multiplication.

For the embodiment of FIG. 4, both pressure regulations are possible through the electrohydraulic shutoff valve 115. Therefore, the adjustment 81 directly correlated with overflow valve 80 is set to a maximum safety value. During the individual mold-filling operations, rest contacts 141 of switch 125 are closed by circuit closer 140 so that there flows, from potential source 145 through exciter winding 150 of shutoff valve 115, the maximum current available from source 145. Accordingly, a maximum pressure, such as is characteristic of electrohydraulic valve 115 at maximum excitation, is blocked. As soon as, at the final mold-filling, the pressure acting on shot piston 62 effects a reversed pressure, determined at pressure In close analogy with the description referring to FIGS. 1

and 2, the embodiment of the invention shown in FIG. 3 will now be explained. In FIG. 3, the pressing unit, comprising the parts 12, 13, and 16 is indicated only marginally, while the pressure medium connections 42, 43, 34 and 36 are represented in a manner substantially identical with the representation in FIG. 2. In the hydraulic control of FIG. 3, a single change has been provided in that, in multiplier control 33, the variation already indicated above, namely the use of a permanently set throttle member together with a very simple pressure vessel with a plain and not stepped piston 106, but with a mechanical stop 98, is employed. Additionally, a pressure-reducing element 108 is arranged between accumulator charging line 34 and pressure vessel 105, in such a way that the pressure supplied to pressure vessel 105 is always dependent reversing member primarily by spring 127 which biases piston 128, closer is shifted from rest contacts 141 to working contacts 142. Initially, time switch 146 is now activated. After expiration of the time set on switch 146, a reduced exciter current flows through exciter winding of valve 115, the current being reduced because potentiometer 147 is now connected in the excitation circuit and serves as the actual excitation regulator. However, this reduced exciter current likewise results in a reduced blocking pressure on precontrol line 82, and this leads to a corresponding reduction of the pressure adjustment of overflow valve 80. Thus, movement of multiplier piston 65 against this adjusted pressure and under the influence of the full pressure of accumulator 16 is initiated. The multiplication thus is controllably fixed.

The embodiment of the invention shown in FIG. 4 has the advantage that, by very simple adjustment of potentiometer 147, any desired multiplication may be set. If desired, several potentiometers may be used so that, for the mold-filling phases as well as for the multiplication, several adjustable already shown in FIGS. 2 and 3,

pressures are available. Instead of using the time switch 146 shown in FIG. 4, an electric condenser in combination with potentiometer 147 may be used, this Combination providing a timed adjustment of the connection of shutoff valve 1 15.

Without going beyond the contextof what has already been described, or without introducing an essentially new idea, it should be pointed out thatthere may be provided, instead of pressure vessel 95 shown in FIG. 2, or instead of the combination ,of pressure vessel 105 and pressure reducing element 108 shown in FIG. 3, whose mechanical separating members 96 and 106, respectively, are hydraulically loaded on both sides, a gas-liquid pressure vessel, whose hydraulic receiving capacity, or gas pressure admission, or both, are adjustable, may be substituted. Thus, the combination, referred to as an R-C member above, of the throttling point 90 with pressure vessel 95 or throttle 90' with pressure vessel 105 is realized in this manner also by the mentioned throttle point and the gas-liquid pressure vessel.

Furthermore, instead of the shutoff valve to be controlled as a function of a pressure difierence, according to FIGS. 2 and 3, there may be provided a valve which, instead of the hydraulic admission originating from the accumulator, presents a selectable mechanical pressure adjustment, in that a practically constant hydraulic pressure admission, on the one hand, and a once selected practically constant pressure adjustment, on the other hand, are equivalent. Against this fixed pressure adjustment, there acts the hydraulic counterpressure control through line 41 and originating from the shot unit.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it should be understood that melts in chill molds, including mold closing means, a filling bush opening into the mold, a pressing ram displaceable in the filling bush and having a hydraulic pressing unit associated therewith, the pressing unit including a shot cylinder, a

hydraulically loadable shot piston displaceable in the shot cylinder, and a multiplier cylinder correlated with the shot cylinder and having a multiplier piston displaceable therein, hydraulically loadable on both sides and hydraulically connectable to the shot piston, an accumulator hydraulically chargeable by a pressure medium source, and hydraulic components interconnecting the pressure mediumsource, the accumulator, the shot cylinder and the multiplier cylinder: the improvement comprising, in combination, a hydraulic line connecting said multiplier cylinder, on that side of said multiplier piston facing said pressure ram, to said pressure medium source; an overflow valve interposed in said hydraulic line and adjustable as to operating pressure; a controllable shutoff valve connected to said overflow valve and acting as a precontrol for said overflow valve, said overflow valve being in the range of influence of the hydraulic pressure effective on that side of said shot piston facing away from said pressing ram; and a pressure dependently operable switching device in controlling connection with said shutoff valve and operative to effect hydraulic connection of said multiplier piston to said shot piston in accordance with the operating pressure of said overflow valve and within the pressure range limited by said accumulator.

2. In a pressure casting machine, the improvement claimed in claim 1, in which said switching device includes means operable to reduce the pressure controlling said shutoff valve, and which is simultaneously efiective on said shot piston, by a predetermined magnitude below the pressure then prevailing in said accumulator.

3. In a pressure casting machine, the improvement claimed in claim 1, in which said switching device controls said shutoff valve with a time delay for reversal of said shutoff valve; a

second hydraulic line connectin the pressure medium admission of said shot piston and sat shutoff valve; throttle means interposed in said second hydraulic line; a branch line connecting said second hydraulic line to said accumulator; a pressure vessel interposed in said branch line; and a mechanical separating member movable in said pressure vessel; said mechanical separating member being subjected to the pressure prevailing in said accumulator as well as to the pressure effective on said shot piston.

4. in a pressure casting machine, the improvement claimed in claim 3, in which said switching device controls said shutoff valve with a variable time delay; said throttle means being adjustable.

5. In a pressure casting machine, the improvement claimed in claim 3, in which said switching device controls said shutoff valve with a variable time delay; said mechanical separating member comprising a piston displaceable in said pressure vessel; and means adjustably limiting movement of said lastnamed piston in said pressure vessel.

6. In a pressure casting machine, the improvement claimed in claim 3, including a pressure reducing member, having a constant pressure drop, interposed in said branch line between said pressure vessel and said accumulator.

7. In a pressure casting machine, the improvement claimed in claim 1, in which said controllable shutoff valve is an electrically controllable shutoff valve; said switching device in cluding a hydraulic pressure operated electric switch controlling said shutoff valve as a function of the pressure effective on said shot piston.

8. In a pressure casting machine, the improvement claimed in claim 7, in which said switching device controls said shutoff valve with a variable time delay; said switching device including an electric time delay circuit in controlling relation with said shutoff valve.

9. in a pressure casting machine, the improvement claimed in claim 7, including at least one electric excitation regulator for said switch-off. valve; said hydraulic pressure operated switch being effective, in one position, to connect said regulator in effective controlling relation with said shutoff valve and, in another position, to disconnect said regulator from effective controlling relation with said shutoff valve; said excitation regulator and said hydraulic pressure operated electric switch providing for the control of at least one overflow pressure to be effected by said shutoff valve in said overflow valve.

10. In a pressure casting machine, the improvement claimed in claim 9, including two adjustable excitation regulators operatively associated with said shutoff valve; said hydraulic pressure operated electric switch being effective to selectively connect a selected one of said adjustable excitation regulators to said shutoff valve; connection of one excitation regulator to said shutoff valve effecting blocking of said multiplier piston, and connection of the other excitation regular to said shutoff valve controlling the magnitude of the multiplier after pressure.

11. In a pressure casting machine, the improvement claimed in claim 1, including a second hydraulic line connecting said shot cylinder, on that side of said shot piston facing said pressure ram, to said pressure medium source; a third hydraulic line connecting said first-mentioned hydraulic line to said second hydraulic line; and a check valve interposed in said third hydraulic line and'blocking flow from said first-mentioned hydraulic line to said second hydraulic line. 

1. In a pressure casting machine, for the casting of metal melts in chill molds, including mold closing means, a filling bush opening into the mold, a pressing ram displaceable in the filling bush and having a hydraulic pressing unit associated therewith, the pressing unit including a shot cylinder, a hydraulically loadable shot piston displaceable in the shot cylinder, and a multiplier cylinder correlated with the shot cylinder and having a multiplier piston displaceable therein, hydraulically loadable on both sides and hydraulically connectable to the shot piston, an accumulator hydraulically chargeable by a pressure medium source, and hydraulic components interconnecting the pressure medium source, the accumulator, the shot cylinder and the multiplier cylinder: the improvement comprising, in combination, a hydraulic line connecting said multiplier cylinder, on that side of said multiplier piston facing said pressure ram, to said pressure medium source; an overflow valve interposed in said hydraulic line and adjustable as to operating pressure; a controllable shutoff valve connected to said overflow valve and acting as a precontrol for said overflow valve, said overflow valve being in the range of influence of the hydraulic pressure effective on that side of said shot piston facing away from said pressing ram; and a pressure dependently operable switching device in controlling connection with said shutoff valve and operative to effect hydraulic connection of said multiplier piston to said shot piston in accordance with the operating pressure of said overflow valve and within the pressure range limited by said accumulator.
 2. In a pressure casting machine, the improvement claimed in claim 1, in which said switching device includes means operable to reduce the pressure controlling said shutoff valve, and which is simultaneously effective on said shot piston, by a predetermined magnitude below the pressure then prevailing in said accumulator.
 3. In a pressure casting machine, the improvement claimed in claim 1, in which said switching device controls said shutoff valve with a time delay for reversal of said shutoff valve; a second hydraulic line connecting the pressure medium admission of said shot piston and said shutoff valve; throttle means interposed in said second hydraulic line; a branch line connecting said second hydraulic line to said accumulator; a pressure vessel interposed in said branch line; and a mechanical separating member movable in said pressure vessel; said mechanical separating member being subjected to the pressure prevailing in said accumulator as well as to the pressure effective on said shot piston.
 4. In a pressure casting machine, the improvement claimed in claim 3, in which said switching device controls said shutoff valve with a variable time delay; said throttle means being adjustable.
 5. In a pressure casting machine, the improvement claimed in claim 3, in which said switching device controls said shutoff valve with a variable time delay; said mechanical separating member comprising a piston displaceable in said pressure vessel; and means adjustably limiting movement of said last-named piston in said pressure vessel.
 6. In a pressure casting machine, the improvement claimed in claim 3, including a pressure reducing member, having a constant pressure drop, interposed in said branch line between said pressure vessel and said accumulator.
 7. In a pressure casting machine, the improvement claimed in claim 1, in which said controllable shutoff valve is an electrically controllable shutoff valve; said switching device including a hydraulic pressure operated electric switch controlling said shutoff valve as a function of the pressure effective on said shot piston.
 8. In a pressure casting machine, the improvement claimed in claim 7, in which said switching device controls said shutoff valve with a variable time delay; said switching device including an electric time delay circuit in controlling relation with said shutoff valve.
 9. In a pressure casting machine, the improvement claimed in claim 7, including at least one electric excitation regulator for said switch-off valve; said hydraulic pressure operated switch being effective, in one position, to connect said regulator in effective controlling relation with said shutoff valve and, in another position, to disconnect said regulator from effective controlling relation with said shutoff valve; said excitation regulator and said hydraulic pressure operated electric switch providing for the control of at least one overflow pressure to be effected by said shutoff valve in said overflow valve.
 10. In a pressure casting machine, the improvement claimed in claim 9, including two adjustable excitation regulators operatively associated with said shutoff valve; said hydraulic pressure operated electric switch being effective to selectively connect a selected one of said adjustable excitation regulators to said shutoff valve; connection of one excitation regulator to said shutoff valve effecting blocking of said multiplier piston, and connection of the other excitation regular to said shutoff valve controlling the magnitude of the multiplier after pressure.
 11. In a pressure casting machine, the improvement claimed in claim 1, including a second hydraulic line connecting said shot cylinder, on that side of said shot piston facing said pressure ram, to said pressure medium source; a third hydraulic line connecting said first-mentioned hydraulic line to said second hydraulic line; and a check valve interposed in said third hydraulic line and blocking flow from said first-mentioned hydraulic line to said second hydraulic line. 